The present invention relates to a developing apparatus for development of a latent image on an image forming retainer for use in an image forming machine, particularly in an electrophotographic image forming machine.
In order to obtain a quality image in the electrophotographic image forming machine using a one-component or two-component developer, it is necessary to form a thin and uniform developer layer on a developer retainer (hereinafter also referred to as the developing sleeve 3, or the sleeve, as shown in FIG. 1, for example).
Conventionally, a developer amount limiting member (hereinafter also referred to as the developer layer limiting member or the developer layer thickness limiting member) has been used for many years constituted of a fixed limiting plate used to equalize the layer thickness. A lower limit of the layer thickness has been approximately 0.3 mm because fastening the developer retainer with the fixed limiting plate has a limited mechanical accuracy. With such a method using the fixed limiting plate it is difficult to obtain a uniform thin layer without unevenness. In addition to the fixed limiting plate, there have been proposed various developer layer limiting devices in order to obtain a uniform thin layer. For example, these include the following:
a. The developer layer forming device disclosed in Japanese Patent Application Laid-Open No. 54-43038/1979 includes a developer layer limiting member made of an elastic plate having a free end which is deflected to press the developer retainer for use with a one-component toner.
b. The developer layer forming device disclosed in Japanese Patent Application Laid-Open No. 54-51848/1979 uses an elastic plate having a metal spring and a soft elastic segment laminated together to make a plate-like surface of which a soft elastic segment presses against a developer retainer to limit the layer thickness of a one-component developer.
c. The developer layer forming device disclosed in Japanese Patent Application Laid-Open Nos. 59-126567/1984 and 59-129879/1984 is to presses an intermittently or continuously rotating elastic roller to a developer retainer to make a nip between the limit layer thickness of a developer in a developing apparatus using a one-component developer.
d. The developer layer forming plate disclosed in the Japanese Patent Publication No. 60-12627/1985 places a rotating roller in contact with a developer retainer of elastic material to limit the layer thickness of a one-component developer in a developing apparatus using a one-component developer.
e. The Japanese Patent Application Laid-Open Nos. 62-191868/1987 and 62-191869/1987 disclose technical means for forming thin developer layers on a developing sleeve 3 useful for non-contact development in order to make the prior devices mentioned above available for two-component developers.
In these disclosures, an end of an elastic plate held by a supporting member is directed toward an upstream portion of a developer on a developing sleeve 3 and is pressed to the developing sleeve 3. This limits the layer thickness of the developer by a magnetic carrier and a toner adhered to and conveyed by the developing sleeve 3. This makes a highly accurate thin developer layer which is easier to form as compared with the previous limiting means.
f. The developing apparatuses using the two-component developer disclosed in the Japanese Patent Application Laid-Open Nos. 61-189582/1986 and 62-75563/1987 have a solid plate-like layer thickness limiting member and a magnetic member on its rear side. The attraction force of a fixed magnet assembly provided in a developer retainer indirectly presses a surface or bent edge of the limiting plate to limit the layer thickness.
g. Japanese Patent Application Laid-Open No. 2-50184/1990 discloses a technique for limiting layer thickness with use of a pressure limiting magnetic bar.
The developer layer forming devices proposed previously involve the following defects. Since both the devices in the prior arts a and b mentioned above use pressure caused by deflection of the elastic layer thickness limiting member, the pressure is likely changed with variation of the rotational speed and pressing position of the developer retainer and the developer layer thickness. Also, since they do not only likely vibrate, but also have no means for suppressing the vibration, they resonate and vibrate with the vibration caused in the image forming apparatus. Thus, it is hard to obtain uniformly the developer layer having a equalized thickness. In particular, the device in the prior art b produces such an adverse effect due to a great extent to pressure applied by the soft elastic member. In addition, a geometrical shape of the pressing member (nip) is likely disturbed by variation of the rotational speed and pressing position of the developer retainer and the developer layer thickness, resulting in a change of the area of the nip. It is a problem that the developer layer thickness is liable to have irregularities. In addition to such difficulties, if one or both of the nips are made of soft elastic material, they become clogged with the developer, and the elastic material wears with time. These difficulties occur to the developer, particularly in the developer containing a magnetic material and fluidizing agent. The device in the prior art c mentioned above can obtain a more stable, uniform thin developer layer than the one in the prior art a, but is inferior in capabilities of demolishing or removing coagulated particles of the developer as its layer limiting means is made of a rotating matter. The coagulated particles are retained by the nip portion and made to pass through with rotation of the rotating matter. This means that the desired performance cannot always be obtained by the intermittent rotation, resulting in image contamination and black spots leading to deterioration of the image. The device is defective as its rotating and pressing arrangements are complicated to balance the pressure.
In any of the prior art devices a to d mentioned above, the layer thickness limiting member has a large pressing area. This adversely causes variation of the developer layer thickness with time corresponding to changes of in the amount of developer fed to the nip portion with time in the ordinary developing apparatus.
The devices in the prior arts c and d are useful for the non-magnetic, one-component developer. They form a thin developer layer in the way that the developer layer thickness limiting member is pressed to the elastic developer retainer. However, they cannot be stable in forming the developer layer as the developer retainer is likely to be permanently deformed, and the developer layer thickness limiting member is changed in elasticity in a long period of operation.
Also, the means in prior art e innovated for the two-component developer cannot be maintained in an operable condition in a long period of operation.
Further, the prior arts a, b, e, and f are to form the thin layer in the way that the elastic developer layer thickness limiting plate is pressed to the stiff developer retainer (metal sleeve). However, they have the disadvantage that the layer thickness limiting plate is short in durability as the elastic plate is likely to change in elasticity and be deformed permanently. Besides, if an installation position of the elastic plate is deviated a little, its end allowance and the pressure are changed. It is hard to stably limit the layer thickness. This is a critical disadvantage in that the installation accuracy is extremely severe in mass production.
The developing apparatus in the prior art f has a feature that the two magnets provided on the rear side of the developer layer thickness limiting member and inside the developer retainer function to demolish the coagulated toner to some degree. However, as this uses attraction through the fixed plate-like limiting member, the attractive force decreases promptly with the distance between both magnets. The pressure, therefore, is likely varied with a change of distance. The pressure effect to the developer layer cannot be stable nor uniform. These lead to the disadvantages that the coagulated toner or developer is made to pass through or conversely, irregular layer thickness causes clogging, resulting in white streaks in the image.
The technique disclosed in the prior art g overcomes most of the disadvantages of the preceding prior arts discussed above, but is not successful and leaves critically important problems unsolved. That is, recent toners are progressively made lower in the softening point in view of use of color toner, energy saving, and low power consumption. In connection to this, the developing sleeve 3 is rubbed with the developer amount limiting member. The toner itself is melted. The melted toner sometime sticks to the developing sleeve 3 and the developer amount limiting member of bar. A cause is that the developing sleeve 3 has irregular, sharp projections on its surface which is generally round. The projections allow conveyance of the developer, but are so sharp that friction heat would be likely generated. Sticking of the toner changes the developer conveyance amount. This causes severe defects, such as concentration unevenness and decrease.
Such drawbacks as described above will not frequently happen and are not normally problematic. But, if it occurs it is a serious difficulty. Such problems can not be allowed to occur often.
In view of the foregoing, it is an object of the present invention to provide a developing apparatus that can form a developer layer thickness on the developer retainer uniformly and stably and can prevent coagulated developer and toner from moving to a developing area, thereby being capable of stably reproducing quality images.